Disculpa te animas a explicarme de nuevo, no te entendí nada!

Buenas dejo una posible solución a mi problema
Código:
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function ShowWindow(ByVal hwnd As IntPtr, ByVal nCmdShow As Int32) As Boolean
    End Function

    Private Sub HideToolStripMenuItem_Click(sender As System.Object, e As System.EventArgs) Handles HideToolStripMenuItem.Click
        Const SW_HIDE As Integer = 0
        Dim p As Process() = Process.GetProcessesByName("firefox")
        If p.Length > 0 Then
            Dim hWnd As IntPtr = p(0).MainWindowHandle
            ShowWindow(hWnd, SW_HIDE)
        End If
    End Sub

Gracias a todos!!